Bonding of leads to integrated circuits has been known since prior to 1972, when U.S. Pat. No. 3,689,991 entitled "Method of Manufacturing a Semi-Conductor Device Utilizing a Flexible Carrier" was issued. This patent deals with tape automated bonding, in which sets of leads carried by a tape are bonded by the application of heat and pressure to small raised "bumps" on the semi-conductor devices. Typically both the leads and the bumps formed on the circuits are made of gold, solder, copper, or other metal or alloy.
A persistent problem with typical bonding of leads to bumps is that since high temperatures and considerable pressures are usually used to form a sound joint, the process tends to create cracks in the layers under the bump. The cracks permit air and moisture to reach the very thin underlying aluminum metallization layer over which the bumps are formed. These layers then oxidize and effectively disappear, allowing the bump to lift off the device and destroying the utility of the device.
The above problem has been known for some time, and various attempts have been made to alleviate it. For example, in a paper entitled "Failure Mechanisms in Tab Inner Lead Bonding and the Relationship Between Design and Reliability" by James D. Hayward, published in IEEE/CHMT '91 IEMT Symposium, page 6, the reliability problems were discussed, and it was suggested that reduction in cracking could be achieved by various design approaches. It was noted that the degree of cracking correlated with the degree of passivation overlap but that neither a reduction in passivation overlap nor a change to make the passivation layer less brittle completely solved the problem. It was suggested that a reduction in the degree of overlap and the use of less brittle passivation layers, as well as the use of appropriate organic die coatings for further sealing, would improve mechanical integrity and reliability. However, it is found that these approaches do not in fact solve the problem.